U.S. patent number 7,207,373 [Application Number 10/973,132] was granted by the patent office on 2007-04-24 for non-oxidizable coating.
This patent grant is currently assigned to United Technologies Corporation. Invention is credited to Gary M. Lomasney, Joseph J. Parkos, Jr., Joshua E. Persky.
United States Patent |
7,207,373 |
Parkos, Jr. , et
al. |
April 24, 2007 |
Non-oxidizable coating
Abstract
A substrate is coated by applying an essentially pure aluminum
first layer to a surface of the substrate. At least a first portion
of the first layer is oxidized so as to provide a protective
coating of desired properties. The substrate may be a refractory
metal-based investment casting core.
Inventors: |
Parkos, Jr.; Joseph J. (East
Haddam, CT), Lomasney; Gary M. (Glastonbury, CT), Persky;
Joshua E. (Carbondale, CO) |
Assignee: |
United Technologies Corporation
(Hartford, CT)
|
Family
ID: |
35241168 |
Appl.
No.: |
10/973,132 |
Filed: |
October 26, 2004 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20060086479 A1 |
Apr 27, 2006 |
|
Current U.S.
Class: |
164/369; 164/24;
164/28; 164/370 |
Current CPC
Class: |
B22C
9/10 (20130101); B22C 9/12 (20130101); C23C
24/04 (20130101); C25D 7/00 (20130101); C25D
11/04 (20130101); C25D 11/18 (20130101); C25D
11/026 (20130101); C23C 28/321 (20130101); C23C
28/322 (20130101); C23C 28/345 (20130101) |
Current International
Class: |
B22C
9/10 (20060101) |
Field of
Search: |
;164/369,370,302,24,28,30,31,32,228 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
European Search Report for EP Patent Application No. 05255424.3
cited by other.
|
Primary Examiner: Kerns; Kevin
Assistant Examiner: Lin; I.-H.
Attorney, Agent or Firm: Bachman & LaPointe, P.C.
Claims
What is claimed is:
1. An investment casting core comprising: a refractory metal-based
substrate; and an essentially chromium-free coating directly atop
the substrate, the coating comprising: a first layer consisting
principally of aluminum oxide, the first layer having a first
thickness in excess of 2.0.mu.; optionally a base layer atop the
substrate and consisting principally of non-oxidized aluminum; and
optionally a transition layer between the first layer and the base
layer.
2. The core of claim 1 wherein: the substrate is
molybdenum-based.
3. The core of claim 1 wherein: the first layer consists
essentially of aluminum oxide and the first thickness is a nominal
first thickness.
4. The core of claim 1 wherein: the first thickness is at least
4.0.mu.; and a combined thickness of the base layer and transition
layer, if either or both are present, is no more than said first
thickness.
5. The core of claim 1 being a first core in combination with: a
ceramic second core; and a hydrocarbon-based material in which the
first core and the second core are at least partially embedded.
6. A plurality of cores of claim 1 in combination with: a natural
or synthetic wax material in which the plurality of cores are at
least partially embedded.
7. An investment casting core comprising: a refractory metal-based
substrate; and an essentially chromium-free coating located
directly atop the substrate, the coating comprising: a first layer
consisting principally of a material in an essentially oxidized
condition, the first layer having a first thickness in excess of
2.0.mu.; a base layer atop the substrate and consisting principally
of said material in an essentially non-oxidized condition; and
optionally a transition layer between the first layer and the base
layer.
8. The core of claim 7 wherein: the substrate is
molybdenum-based.
9. The core of claim 7 wherein: the material comprises an aluminum
alloy.
10. The core of claim 9 wherein: the aluminum alloy comprises 0.25%
1.0 weight percent of one or a combination of Ca, Mg, Si, and
Zr.
11. The core of claim 9 wherein: the aluminum alloy comprises 0.25%
1.0 weight percent of Mg.
12. The core of claim 7 wherein: the first layer comprises
principally .alpha.-phase.
13. The core of claim 7 wherein: the material comprises an
aluminum-silica alloy.
14. The core of claim 7 wherein: the first thickness is at least
4.0.mu.; and the base layer has a second thickness in excess of
2.0.mu..
15. The core of claim 7 being a first core in combination with: a
ceramic second core; and a hydrocarbon-based material in which the
first core and the second core are at least partially embedded.
16. A method for forming an investment casting core comprising:
applying an initial layer of a first material to a surface of a
substrate of a second material different from the first material;
and oxidizing at least a first portion of the initial layer so as
to leave a principally oxidized sublayer of at least 5.0.mu. and an
essentially intact sublayer of the first material of at least
2.0.mu..
17. The method of claim 16 wherein the initial layer comprises, in
major weight part, one or more of: Al; Ca; Mg; Si; and Zr.
Description
BACKGROUND OF THE INVENTION
The invention relates to metallic coating. More particularly, the
invention relates to protective coating of oxidizable investment
casting cores.
Investment casting is a commonly used technique for forming
metallic components having complex geometries, especially hollow
components, and is used in the fabrication of superalloy gas
turbine engine components.
Gas turbine engines are widely used in aircraft propulsion,
electric power generation, and ship propulsion. In gas turbine
engine applications, efficiency is a prime objective. Improved gas
turbine engine efficiency can be obtained by operating at higher
temperatures, however current operating temperatures in the turbine
section exceed the melting points of the superalloy materials used
in turbine components. Consequently, it is a general practice to
provide air cooling. Cooling is provided by flowing relatively cool
air from the compressor section of the engine through passages in
the turbine components to be cooled. Such cooling comes with an
associated cost in engine efficiency. Consequently, there is a
strong desire to provide enhanced specific cooling, maximizing the
amount of cooling benefit obtained from a given amount of cooling
air. This may be obtained by the use of fine, precisely located,
cooling passageway sections.
A well developed field exists regarding the investment casting of
internally-cooled turbine engine parts such as blades and vanes. In
an exemplary process, a mold is prepared having one or more mold
cavities, each having a shape generally corresponding to the part
to be cast. An exemplary process for preparing the mold involves
the use of one or more wax patterns of the part. The patterns are
formed by molding wax over ceramic cores generally corresponding to
positives of the cooling passages within the parts. In a shelling
process, a ceramic shell is formed around one or more such patterns
in well known fashion. The wax may be removed such as by melting in
an autoclave. The shell may be fired to harden the shell. This
leaves a mold comprising the shell having one or more part-defining
compartments which, in turn, contain the ceramic core(s) defining
the cooling passages. Molten alloy may then be introduced to the
mold to cast the part(s). Upon cooling and solidifying of the
alloy, the shell and core may be mechanically and/or chemically
removed from the molded part(s). The part(s) can then be machined
and treated in one or more stages.
The ceramic cores themselves may be formed by molding a mixture of
ceramic powder and binder material by injecting the mixture into
hardened steel dies. After removal from the dies, the green cores
are thermally post-processed to remove the binder and fired to
sinter the ceramic powder together. The trend toward finer cooling
features has taxed core manufacturing techniques. The fine features
may be difficult to manufacture and/or, once manufactured, may
prove fragile. Commonly-assigned co-pending U.S. Pat. No. 6,637,500
of Shah et al. discloses general use of refractory metal cores in
investment casting among other things. Various refractory metals,
however, tend to oxidize at higher temperatures, e.g., in the
vicinity of the temperatures used to fire the shell and the
temperatures of the molten superalloys. Thus, the shell firing may
substantially degrade the refractory metal cores and, thereby
produce potentially unsatisfactory part internal features. Use of
protective coatings on refractory metal core substrates may be
necessary to protect the substrates from oxidation at high
temperatures. An exemplary coating involves first applying a layer
of chromium to the substrate and then applying a layer of aluminum
oxide to the chromium layer (e.g., by chemical vapor deposition
(CVD) techniques). However, particular environmental/toxicity
concerns attend the use of chromium. Accordingly, there remains
room for further improvement in such coatings and their application
techniques.
SUMMARY OF THE INVENTION
One aspect of the invention involves an investment casting core
having a refractory metal-based substrate and an essentially
chromium-free coating directly atop the substrate. The coating
includes a first layer consisting principally of aluminum oxide.
The first layer has a first thickness in excess of 2.0.mu..
Optionally, a base layer may be located atop the substrate and
consist principally of non-oxidized aluminum. Optionally, a
transition layer may be located between the first layer and the
base layer.
In various implementations, the substrate may be molybdenum-based.
The first layer may consist essentially of aluminum oxide and the
first thickness may be a nominal (e.g., a median) first thickness.
The first thickness may be at least 4.0.mu.. A combined thickness
for the base layer and transition layer, if either or both are
present, may be no more than the first thickness. The core may be a
first core in combination with a ceramic second core and a
hydrocarbon-based material in which the first core and the second
core are at least partially embedded.
Another aspect of the invention involves a method for coating a
substrate. An essentially pure aluminum initial layer is applied to
a surface of the substrate. At least a first portion of the initial
layer is oxidized so as to leave the first portion with an
unoxidized aluminum content of no more than 10% of a total aluminum
content and a thickness of at least 2.0.mu..
In various implementations, the applying may form the initial layer
with a characteristic thickness of about 25.mu. 75.mu.. The
applying may include at least one of ion vapor deposition, cold
spray, and electrolytic deposition. The applying may consist
essentially of ion vapor deposition. The oxidizing may include at
least one of anodizing, hard coating, and micro-arc oxidation. The
substrate may include at least one of a refractory metal-based
material, an aluminum alloy, and a non-metallic composite. The
substrate may consist essentially of a molybdenum-based material.
The oxidizing may oxidize a majority of the aluminum in the applied
initial layer. The method may be used to form an investment casting
core component.
The method may further include assembling the core with a second
core. A sacrificial material may be molded to the core and second
core. A shell may be applied to the sacrificial material. The
sacrificial material may be essentially removed. The metallic
material may be cast at least partially in place of the sacrificial
material. The core, second core, and shell may be destructively
removed. Alternatively, the second core may be formed at least
partially over the core.
Another aspect of the invention involves an article having a
substrate having an essentially chromium-free surface. An
essentially chromium-free coating is located directly atop the
surface. The coating includes a first layer consisting essentially
of aluminum oxide. The first layer has a first thickness in excess
of about 2.0.mu.. Optionally, a base layer may be located directly
atop the surface and consist essentially of non-oxidized aluminum.
Optionally, a transition layer may be located between the first
layer and the base layer.
In various implementations, the substrate may be molybdenum-based.
The first layer may the first layer may have a density of at least
3.4 g/cc and a principally .alpha.-phase microstructure. The first
layer may have a density of 3.6 4.0 g/cc and an essentially
.alpha.-phase microstructure.
The details of one or more embodiments of the invention are set
forth in the accompanying drawings and the description below. Other
features, objects, and advantages of the invention will be apparent
from the description and drawings, and from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a shelled investment casting
pattern for forming a gas turbine engine airfoil element.
FIG. 2 is a sectional view of a refractory metal core of the
pattern of FIG. 1.
FIG. 3 is a flowchart of processes for forming and using the
pattern of FIG. 1.
Like reference numbers and designations in the various drawings
indicate like elements.
DETAILED DESCRIPTION
FIG. 1 shows a shelled investment casting pattern 20 including a
pattern 22 and a ceramic shell 24. The pattern 22 includes a
sacrificial wax-like material 26 (e.g., natural or synthetic wax or
other hydrocarbon-based material) at least partially molded over a
core assembly. The core assembly includes a ceramic feed core 28
having a series of generally parallel legs 30, 32, and 34 for
forming a series of generally parallel, spanwise-extending, feed
passageways in the ultimate part being cast (e.g., a gas turbine
engine turbine blade, or vane). Assembled to the feed core 28 are a
series of refractory metal cores (RMCs) 36 and 38. Portions of the
RMCs 36 and 38 may be received in compartments 40 and 42 in the
feed core 28 and secured therein via ceramic adhesive 44. Other
portions of the RMCs 36 and 38 may be embedded in the shell 24 so
that the RMCs 36 and 38 ultimately form outlet passageways from the
feed passageways to the exterior surface of the part. The exemplary
RMCs 36 provide film cooling passageways for airfoil pressure and
suction side surfaces and the exemplary RMC 38 provides airfoil
trailing edge cooling. Many other configurations are possible
either in the prior art or yet to be developed.
FIG. 2 shows further details of one of the RMCs (e.g., 38). The
exemplary RMC 38 has a substrate 50 of refractory metal or a
refractory metal-based alloy, intermetallic, or other material.
Exemplary refractory metals are Mo, Nb, Ta, and W. These may be
obtained as wire or sheet stock and cut and shaped as appropriate.
A coating system includes an aluminum first layer 52 atop the
substrate and an aluminum oxide (alumina) second layer 54 atop the
first layer 52. It is believed that .alpha.-phase alumina offers
advantageous hardness and adhesion/retention over a broad
temperature range. Nevertheless other phases (e.g., material
comprising or consisting essentially of one or both of .beta.- and
.gamma.-phase) may be used. Exemplary alumina density is 3.4 4.0
g/cc
The exemplary substrate 50 is formed, e.g., from sheet stock having
a surface including a pair of opposed faces 56 and 58 with a
thickness T between. Complex cooling features may be stamped, cut,
or otherwise provided in the substrate 50. An interior surface 60
of the coating system and first layer 52 sits atop the exterior
surface of the substrate 50 and an exterior surface 62 of the
coating system and second layer 54 provides an exterior surface of
the RMC 38. A transition 64 separates the first layer 52 from the
second layer 54. The transition 64 may be fairly abrupt or may be a
transition region characterized by a compositional median or
compositional gradation. In the exemplary embodiment, the coating
system has a thickness T.sub.1, the first layer 52 has a thickness
T.sub.2, and the second layer 54 has a thickness T.sub.3.
FIG. 3 shows an exemplary process 200 of manufacture and use
(simplified for illustration). The substrate(s) are formed 202 such
as via stamping from sheet stock followed by subsequent bending or
other forming to provide a relatively convoluted shape for casting
the desired features. An essentially pure aluminum coating is
deposited 204 atop the substrate. The deposition process may be a
physical or chemical deposition process. Exemplary physical
deposition processes are ion vapor deposition (IVD) and cold spray
deposition. Exemplary IVD and cold spray deposition techniques are
shown in U.S. Military Standard Mil-C-83488 (for pure Al) and U.S.
Pat. No. 5,302,414 of Alkhimov et al., respectively. Exemplary
chemical processes include electrolytic plating. The deposited
aluminum layer is then at least partially oxidized 206 to form the
second layer 54 and leave the first layer 52. Exemplary oxidation
is via chemical process such as anodizing, hard coating (a family
of high voltage anodizing processes), and micro-arc oxidation.
Exemplary micro-arc processes are shown in U.S. Pat. Nos.
6,365,028, 6,197,178, and 5,616,229.
The RMCs are then assembled to the feed core(s) which may be formed
separately 210 (e.g., by molding from silicon-based material) or
formed as part of the assembling (e.g., by molding the feed core
partially over the RMCs). The assembling may also occur in the
assembling of a die for overmolding 212 the core assembly with the
wax-like material 26. The overmolding 212 forms a pattern which is
then shelled 214 (e.g., via a multi-stage stuccoing process forming
a silica-based shell). The wax-like material 26 is removed 216
(e.g., via steam autoclave). After any additional mold preparation
(e.g., trimming, firing, assembling), a casting process 218
introduces one or more molten metals and allows such metals to
solidify. The shell is then removed 220 (e.g., via mechanical
means). The core assembly is then removed 222 (e.g., via chemical
means). The as-cast casting may then be machined 224 and subject to
further treatment 226 (e.g., mechanical treatments, heat
treatments, chemical treatments, and coating treatments).
The coating process may provide an initial aluminum thickness in
the range of 0.25 5 mil (6 130.mu.), more preferably. 1 3 mil (25
75.mu.). Some of this material is then oxidized to form the second
layer 54. During the oxidation, some of the aluminum may be lost
(e.g., into the anodizing bath). Advantageously, little if any of
the aluminum diffuses into the substrate at least until
firing/casting. At those elevated temperatures, some or all of the
theretofore unoxidized aluminum may diffuse into/with the substrate
material. The oxidation may advantageously form the second layer
with the thickness T.sub.3 in the vicinity of 5.mu. or more to
provide adequate insulation. More broadly, the thickness may be in
excess of 2.mu. (e.g., 4.mu. 50.mu., or 20 40.mu.). Advantageously,
at least 90% of the aluminum in the second layer 54 may be
oxidized. The oxidation tends to expand the thickness of the second
layer by 100% relative to the thickness of the deposited aluminum
being oxidized. Thus, in the absence of diffusion or loss, a 25.mu.
deposited aluminum layer could, if oxidized across its thickness,
produces an aluminum oxide layer of thickness in the vicinity of
50.mu.. With a 20% loss and oxidation across substantially half the
depth, the remaining first layer thickness T.sub.2 would be about
10.mu. and the aluminum oxide second layer thickness T.sub.3 would
be about 20.mu.. The foregoing numbers are merely exemplary.
Advantageously, however, at least with the exemplary molybdenum
substrate and various annodization processes, the first layer
thickness is at least about 2.0.mu.. That is the minimum thickness
believed appropriate to isolate the substrate from the effects of
the annodization. If the thickness T.sub.2 becomes less, the
molybdenum may begin to dissolve, destroying the coating adherence.
There is no inherent upper limit to the thickness T.sub.2. However,
excess thickness poses cost issues and represents a loss of
insulation contrasted with the situation where such excess material
is converted to alumina. Thus, typically, the alumina thickness
T.sub.3 will be at least half the total coating thickness
T.sub.1.
The coating technique may have broader applicability. For example,
the substrate may be of highly alloyed aluminum atop which the
purer aluminum layer is deposited and then at least partially
oxidized. Alternatively, the substrate may be a composite
material.
Various dopants or alloying elements may be used. Ca, Mg, Si, and
Zr, for example, form stable oxide systems CaO, MgO, SiO.sub.2,
ZrO.sub.2. These elements or their combinations may be deposited in
an alloy with the aluminum to be oxidized (e.g., in exemplary low
quantities of less than 1% by weight to control grain growth and
the morphology of the coating and influence properties such as
CTE). Greater quantities of these elements (including even major
portions of the as-applied coating--pre-oxidation) are
possible.
The present system and methods may have one or more advantages over
chromium-containing coatings. Notable is reduced toxicity. Chromium
containing coatings are typically applied using solutions of
hexvalent chromium, a particularly toxic ion. Furthermore, when the
coated core is ultimately dissolved, some portion of the chromium
will return to this toxic valency. The present coatings may have
less than 0.2%, preferably less than 0.01% chromium by weight, and,
most preferably, no detectable chromium. The present system and
methods may have one or more advantages over single-step coating of
a substrate (e.g., molybdenum) with aluminum oxide. The aluminum
oxide layer may have higher density. A greater evenness may be
obtainable by using aluminum deposition techniques that do not
suffer from the same line-of-sight problems as various single-step
aluminum oxide deposition techniques.
One or more embodiments of the present invention have been
described. Nevertheless, it will be understood that various
modifications may be made without departing from the spirit and
scope of the invention. For example, the coatings may be utilized
in the manufacture of cores of existing or yet-developed
configuration. The details of any such configuration may influence
the details of any particular implementation as may the details of
the particular ceramic core and shell materials and casting
material and conditions. Accordingly, other embodiments are within
the scope of the following claims.
* * * * *